add a packageImports fields to the analysis response

[devoncarew]

• add a packageImports fields to the analysis response

We can use this field to help know whether to compile with DDC or dart2js.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[devoncarew]

I'm going to TBR this so that field is available on the live backends, but please feel free to do a post-merge review.

[mosuem]

(Nit) Why check this when checking for flutter in the next line?

[devoncarew]

Yeah, its redundant; the intent was to test that we parsed some imports, and then that we parsed the correct imports, but we could just as well only do the 2nd test.